1. Field of the Invention
This invention relates to a fluorescent display device, and more particularly to a fluorescent display device of the front emission type wherein luminous display is observed through one surface of a light-permeable insulating substrate which has a phosphor-coated anode conductor formed on the other surface thereof.
2. Description of the Prior Art
In a fluorescent display device of the front emission type, a wiring conductor and an anode conductor each are prepared in a manner to form a film of a conductive material on a substrate by a suitable means such as vacuum deposition, sputtering or the like and remove an unnecessary portion from the film by photolithography to form a predetermined pattern. Thus, the wiring conductor and anode conductor are typically formed of the same electrically conductive material.
A fluorescent display device is generally classified into a fluorescent display device wherein a conductive material is formed of a transparent conductive film such as an ITO (indium-tin-oxide) film, a tin oxide film or the like and a fluorescent display device wherein a conductive material comprises a metallic film and an anode conductor is formed into a mesh-like or pectinate shape to define gaps therein.
The present invention is generally directed to an improvement in an anode substrate for a fluorescent display device of the front emission type wherein a conductive material is formed of a metallic film.
One of conventional fluorescent display devices of the front emission type is illustrated in FIGS. 1 and 2 which employs a dynamic driving system. In the fluorescent display device shown in FIGS. 1 and 2, wiring conductors 104 are formed by forming a film of aluminum on a substrate 101 and remove an unnecessary portion from the film by etching utilizing photolithography to form slit sections 102 to divide the film every display segment group 103. Each of the wiring conductors 104 has corresponding display segments of the respective digits connected thereto. The segments each are provided thereon with a slit or mesh-like anode conductor 105, on which a phosphor layer 106 is deposited to cover at least an opening of each display segment. For exaple, in FIG. 1, the phosphor layer 106 is deposited to extend to the periphery of the opening as shown in dotted lines.
Another one of conventional fluorescent display devices of the front emission type is shown in FIG. 3 in which a static driving system is employed. In the fluorescent display device shown in FIG. 3, a control electrode is eliminated therefrom and an auxiliary electrode is substitutionally provided on the same plane as an anode so as to prevent charging at the periphery of the anode and uniformly accelerate electrons emitted from a cathode.
In the fluorescent display device of FIG. 3, the formation of wiring conductors is carried out by applying a film of aluminum onto a substrate 101 and removing an unnecessary portion of the film by etching utilizing photolithography to form slit sections 102, to thereby form fine wiring conductors 104 isolated from a metallic film 107 of the background and terminal connections 104a positioned at one end thereof. Each of segments 103 is formed at the outer periphery thereof with a frame 108 which is connected to the corresponding wiring conductor 104. A mesh-like anode conductor 105 arranged within the frame 108 is formed in the substantially same manner as in FIG. 1. A phosphor layer 106 is deposited on the anode conductor 105 to cover an opening P of each display segment and the frame 108 positioned at the periphery thereof as indicated in dotted lines in FIG. 3. However, even in this instance, it is merely required to deposit the phosphor layer 106 so as to cover at lesat the opening P of the display segment.
The fluorescent display device shown in FIG. 3, as described above, is constructed to isolate the metallic film 107 constituting the background from the anode conductors 104 and wiring conductors 105 so that potential constantly positive with respect to a cathode may be applied to the metallic film 107 to allow constantly positive electric field to be formed around the anode, to thereby effectively prevent any display defect.
When the anode substrate substantially covered with the film of aluminum as described above is viewed from the substrate side, the wiring conductors 104 and mesh-like anode conductors 105 formed of the aluminum film are observed as a specular surface through the substrate 101 as shown in FIG. 2. The specular surface causes external light to be reflected thereby at a specular reflection ratio of about 80% to hinder the reading of the display segments, to thereby decrease in visibility.
An attempted solution to such a problem as described above has been to provide a fluorescent display device which is shown in FIGS. 4 and 5 and adapted to reduce the area of a film pattern section, to thereby decrease the specular reflection. In the fluorescent display device of FIGS. 4 and 5, anode conductors 202 and wiring conductors 203 are formed by forming a film of aluminum on the overall surface of a substrate 201 by a suitable method such as vacuum deposition, sputtering or the like and then removing an unnecessary portion from the film by etching utilizing photolithography to leave the anode conductors 202 and wiring conductors 203. The anode conductors 202 each are constituted by a frame-like anode conductor 202a and a mesh-like anode conductor 202b. The wiring conductors 203 each are formed by removing an unnecessary portion from the aluminum film by etching utilizing photolithography to form a predetermined pattern so that it has one end connected to the corresponding anode conductor 202 and the other end formed with a terminal connection 204.
The substrate 201 is formed on the portion thereof except the anode conductors 202 with a colored insulating layer 205. The insulating layer 205 is colored black so as to reduce reflection of external light. A phosphor layer 206 is deposited on each of the mesh-like anode conductors 202b and the periphery thereof. Such deposition may be carried out in a manner to extend a part of the phosphor layer 206 to the frame-like anode conductor 202a, as shown in FIG. 5.
Nevertheless, even the fluorescent display device shown in FIGS. 4 and 5 which is constructed in the manner as described above still has a disadvantage that the frame-like anode conductors 202a, wiring conductors 203, terminal connections 204 and the like are observed as a specular surface as shown in FIG. 4 when luminous display is viewed through the substrate 201. Further, the fluorescent display device has another disadvantage that the manufacturing process and construction are complicated due to use of the insulating layer 205. A further problem of the fluorescent display device is that there are encountered deformation of the substrate, consumption of the aluminum film and the like due to the calcination of the insulating layer 205 at a high temperature.